1. Field of the Invention
This invention relates to curable organopolysiloxane compositions comprising silicon-bonded 3,3,3-trifluoropropyl groups and to a method for injection molding of said composition to form an elastomeric article.
2. Description of the Prior Art
Injection molding of liquid, curable compositions to form small elastomeric articles is highly desirable because the attendant low injection pressures and rapid production rates permit a greater selection of injecting and molding conditions and greater economy. It is desirable to use organopolysiloxane compositions bearing 3,3,3-trifluoropropyl groups for the preparation of certain elastomeric articles since said compositions, when cured, display usefulness in extremely cold or hot environments in the presence of many solvents. However, prior art organopolysiloxane compositions bearing 3,3,3-trifluoropropyl groups are unsatisfactory in many respects for the injection molding of small elastomeric articles.
Organopolysiloxane compositions wherein essentially all of the polymer units are methyl-3,3,3-trifluoropropylsiloxane units are commercially available in many forms. In the cured form these compositions possess excellent resistance to the deleterious effects of hydrocarbon fuels and oils, but their resistance to polar organic compounds such as hydrocarbon esters and ketones is much less. These compositions are thus unsatisfactory for the preparation of articles for general applications.
It has been shown by Holbrook, Canadian Pat. No. 571,381 to prepare vulcanizable compositions comprising a filler, a vulcanizing agent and a copolymer of a fluorinated siloxane and a diorganosiloxane, wherein the non-fluorinated organic groups of the copolymer are monovalent hydrocarbon radicals. However, these compositions have inferior physical properties for many applications.
Curable compositions comprising a vinyl-containing polydiorganosiloxane and a siloxane bearing silicon-bonded hydrogen atoms wherein the latter siloxane comprises molecules having two silicon-bonded hydrogen atoms and molecules having three or more silicon-bonded hydrogen atoms are known. Thus, Polmanteer, et al., U.S. Pat. No. 3,697,473 teach a curable composition consisting essentially of a triorganosiloxy-terminated polydiorganosiloxane having two vinyl radicals per molecule and having a viscosity of from 100 to 10,000 centipoise at 25.degree. C., where at least 50 percent of the organic radicals are methyl, and a mixture of silicon-containing compounds having silicon-bonded hydrogen atoms. The mixture of silicon-containing compounds having silicon-bonded hydrogen atoms consists essentially of an organosiloxane compound containing two silicon-bonded hydrogen atoms per molecule and an organosiloxane compound containing from 3 to 10 inclusive silicon-bonded hydrogen atoms per molecule. At least 10 percent of the silicon-bonded hydrogen atoms are derived from each of the two types of organosiloxane compounds bearing silicon-bonded hydrogen atoms. The silicon-bonded hydrogen atoms in any of the organosiloxane compounds can be of the type HSiO.sub.3/2 or the hydrogen atoms can be bonded to silicon atoms bearing one or two organic radicals. However, Polmanteer, et al. do not teach or suggest the unique features of a curable composition comprising a copolymer of dimethylsiloxane units and methyl-3,3,3-trifluoropropylsiloxane units that is suitable for liquid injection molding of small elastomeric articles.
To be useful for liquid injection molding of small elastomeric articles a curable composition should have a high injection rate and a short cure time; and the cured composition should have sufficient strength to allow the removal of the hot, elastomeric article from a hot mold. No prior art organopolysiloxane composition comprising a copolymer consisting essentially of dimethylsiloxane units and methyl-3,3,3-trifluoropropylsiloxane units is useful for such a liquid injection molding process. While it is well known to prepare a curable organopolysiloxane composition by mixing a vinyl-endblocked polydiorganosiloxane, a treated filler, a curing agent bearing silicon-bonded hydrogen atoms, a curing catalyst and an inhibitor for the catalyst, the unique combination of certain components to give the compositions for this invention is neither known nor obvious.